Weapons systems have been developed that include reentry bodies that with guidance and navigation systems to control reentry of the body after separation from a launch vehicle. To accurately control the trajectory of the reentry body, the guidance and navigation system of the reentry body uses position, velocity and orientation information. Typically, the reentry body has an inertial measurement unit (IMU) to provide data to the guidance and navigation system during reentry.
The launch vehicle typically also has a guidance and navigation system along with appropriate sensors, e.g., an IMU, etc. The IMU in the launch vehicle has its own reference frame; commonly called the inertial frame (I-frame). Similarly, the IMU in the reentry vehicle has its own reference frame; commonly referred to as the pseudo-inertial frame (P-frame). To allow the reentry vehicle to properly navigate after separation from the launch vehicle, flight systems commonly determine the orientation of the P-frame with respect to the I-frame using, e.g., a Kalman filter. The Kalman filter outputs information used by the reentry body's navigation computer to determine the vehicle position and velocity in the reference I-frame.
Unfortunately, the Kalman filter is complex and difficult to implement. Specifically, the Kalman filter requires an initial estimate of the relative orientations of the I-frame and P-frame such that the small angle approximation is valid. A poor initial estimate could lead to divergence of the Kalman filter even when there is complete observability of the relative orientations.
Therefore, there is a need in the art for an alignment mechanism in a flight vehicle that does not require an a priori estimate that is good enough for the small angle approximation to be valid.